An optical phantom is a fabricated sample that provides an optical response similar to biological tissues for examination by one or more optical imaging system. In many cases, the interaction of electromagnetic (EM) radiation (herein “light”) with a tissue is described by the scattering and the absorption processes. In a scattering process, light is essentially redirected in a different direction. In the absorption process, light is absorbed and energy is converted into a different form. Therefore, phantoms are often made of scattering and/or absorbing materials or mixtures that can produce the desired response. In the literature, one can find a variety of phantom fabrication processes with different materials that provide optical responses somewhat similar to tissues. The main differences between the resulting phantoms are often in terms of other important properties, like mechanical properties or durability.
Many phantom compositions are made of liquid or gel. These phantoms suffer from a conservation period limited to months in the best cases due to perishing or water evaporation (U.S. Pat. No. 7,288,759 to Frangioni et al.). A housing is sometimes used to increase durability but can be inconvenient in use because such housings are known to influence measurements (U.S. Pat. No. 6,675,035 to Grable et al.).
Phantoms that are durable for years can be obtained with polymeric matrices such as polyester, epoxy resins or dried poly(vinyl alcohol) mixed with inorganic components (U.S. Pat. No. 6,083,008 to Yamada et al., and U.S. Pat. No. 6,224,969 to Steenbergen). However, these matrices are hard and do not provide mechanical properties similar to soft tissues. This limits some uses of phantoms, for example in training surgeons on operations, as haptic and tactile responses are not similar, especially for procedures like endoscopy.
Phantoms with elastomeric matrices, like silicone, combine durability with mechanical properties somewhat similar to soft tissues. These were presented in a number of publications reviewed by Pogue and Patterson, in Journal of Biomedial Optics, 11 (4), 041102, (2006). The mechanical properties can also be adjusted by modifying the silicone formulation (Oldenburg et al. in Optic Letters, 30 (7), 747, (2005) and U.S. Pat. No. 7,419,376 to Sarvazyan). Some optical properties can be obtained by introducing inorganic powders in the silicone matrix. An experimental calibration can be conducted to relate the powder concentrations and the optical properties, like by Beck et al. in Lasers in Medical Science, 13 (3), 160 (1998) and by Lualdi et al. in Lasers in Surgery and Medicine, 28, 237, (2001). Some slab shape phantoms using different mixtures representing multiple skin layers and lesions have also been published, for example by Urso et al, in Physics in Medicine and Biology, 52 (10), N229, (2007).
Very few phantoms have been designed for endoscopic applications. Endoscopic optical applications are increasing with the development of specialized optical probes that are able to deliver light to internal organs using optical fibers. Many of these organs, like blood vessels, bronchi, the esophagus, the colon, etc. have openings of somewhat cylindrical or tubular geometries, or define somewhat closed cavities. Herein a lumen is used to refer to a tubular or a closed cavity that is formed of walls on two or three sides. Such walls generally include tissues built up in layers, each of which having different composition, function, and optical and mechanical properties.
Silicone-based phantoms with complex geometries have been molded in various shapes (U.S. Pat. No. 6,807,876 and Bays et al. in Lasers in Surgery and Medicine, 21(3), 227, (1997)) but do not show a detailed layered structure. A molding process limits the shapes and the dimensions of the phantoms to the ones of the available molds. Furthermore molding of very thin layers, less than about 25 μm for example, can be exceedingly difficult, generally requires high pressure (which can damage other delicately formed features) and is prone to failure.
Further still, it is generally desired to provide phantoms that contain inclusions such as features that optically and/or mechanically represent lesions, tumors, scar tissues, inclusions, herniations etc. While highly planar features can be readily provided, even between layers of a phantom, by application of paint or powder prior to a subsequent overmolding step, the embedding of solid objects within a mold can be exceedingly difficult. A general failure to provide precise localization of the object within a mold and numerous defects are recurring problems with solid objects in molds. These problems may be exacerbated by subsequent overmolding steps.
Finally it is very difficult to produce very uniform thickness layers, or to provide a very high measure of control of thickness as a function of position of the layer, unless the phantom is molded. With multilayered structures, the costs and tolerances of many molds that fit inside one-another is prohibitive for many applications.
There remains a need for multilayer optical phantoms representing animal tissues and organs containing lumens, and for methods of fabricating same, especially for fabrication methods that permit precise control of the layer thicknesses, down to layers of the order of 10 μm, and permitting solid body inclusions between the layers.